Crystal growth method for nitride semiconductor and formation method for semiconductor device

ABSTRACT

Methods of crystal growth for semiconductor materials, such as nitride semiconductors, and methods of manufacturing semiconductor devices are provided. The method of crystal growth includes forming a number of island crystal regions during a first crystal growth phase and continuing growth of the island crystal regions during a second crystal growth phase while bonding of boundaries of the island crystal regions occurs. The second crystal growth phase can include a crystal growth rate that is higher than the crystal growth rate of the first crystal growth phase and/or a temperature that is lower than the first crystal growth phase. This can reduce the density of dislocations, thereby improving the performance and service life of a semiconductor device which is formed on a nitride semiconductor made in accordance with an embodiment of the present invention.

RELATED APPLICATION DATA⁻²

[0001] The present application claims priority to Japanese PatentDocument No. P2001-113713 herein incorporated by reference to the extentpermitted by law.

BACKGROUND OF THE INVENTION

[0002] The present invention generally relates to methods of crystalgrowth on a semiconductor material. More specifically, the presentinvention relates to crystal growth methods for a nitride semiconductor,such as a gallium nitride based semiconductor and a methods for formingsemiconductor devices employing crystal growth methods of the presentinvention capable of fabricating a variety of semiconductor devicesincluding, for example, a semiconductor light emitting device, such as asemiconductor light emitting diode, a semiconductor laser, asemiconductor transistor device or the like.

[0003] In general, known vapor-phase growth techniques for a nitridesemiconductor, such as a gallium nitride based compound semiconductor,can be problematic as it is difficult to obtain a substrate beinglattice matched with a nitride semiconductor or a substrate having a lowdensity of dislocations. To solve such a problem, there has been known atechnique of depositing a low temperature buffer layer made from AIN orAl_(x)Ga_(1−x)N (0≦×<1) at a low temperature of 900° C. or less on asurface of a substrate made from sapphire or the like, and then growinga gallium nitride based compound semiconductor thereon, thereby reducingdislocations due to lattice mismatching between the substrate and thecompound semiconductor. Such a technique has been disclosed, forexample, in Japanese Patent Laid-open No. Sho 63-188938 and JapanesePatent Publication No. Hei 8-8217. By using such a technique, it ispossible to obtain a gallium nitride based compound semiconductor withimproved crystallinity and morphology.

[0004] Another technique of obtaining high quality crystal at a lowdensity of dislocations has been disclosed, for example, in JapanesePatent Laid-open Nos. Hei 10-312971 and Hei 11-251253. This methodinvolves depositing a first gallium nitride based compound semiconductorlayer, forming a protective film made from a material capable ofinhibiting growth of a gallium nitride based compound semiconductor,such as silicon oxide or silicon nitride, in such a manner as toselectively cover the first gallium nitride based compoundsemiconductor, and growing a second gallium nitride based compoundsemiconductor in an in-plane direction (lateral direction) from regions,not covered with the protective film, of the first gallium nitride basedcompound nitride layer, thereby preventing propagation ofthrough-dislocations extending in the direction perpendicular to theinterface of the substrate.

[0005] A further technique of reducing a density of through-dislocationshas been disclosed, for example, in MRS Internet J. Nitride Semicond.Res. 4S1, G3. 38 (1999). This method involves growing a first galliumnitride based compound semiconductor, selectively removing the thusformed semiconductor film by using a reactive ion etching (hereinafter,referred to as “RIE”) system, and selectively growing a second galliumnitride based compound semiconductor from the remaining crystal in thegrowth apparatus. According to this method, it is possible to obtain acrystal film having a density of dislocations, which is reduced to about10⁶/cm², and hence to realize a high life semiconductor laser using thecrystal film formed according to this method.

[0006]FIGS. 8A to 8D are sectional views showing steps of one relatedart crystal growth method for a gallium nitride based compoundsemiconductor. Referring to FIG. 8A, after a low temperature bufferlayer is formed on a C-plane 101 of a sapphire substrate 100, the supplyof trimethyl gallium is stopped while the supply of ammonia iscontinued, with a result that grains each having a size on order ofseveral tens of nanometers, which are nuclei for formation of a galliumnitride layer 102, are formed. Referring to FIG. 8B, as the supply oftrimethyl gallium begins again, crystal growth occurs from the grains,to form island crystal regions each laterally extending on the C-plane101. When the crystal growth proceeds at a crystal growth rate of 4 μm/h(which crystal growth rate is a value converted into a crystal growthrate in film-like crystal growth on a plane), boundaries of the islandcrystal regions are bonded to each other as shown in FIG. 8C, andfurther, a gallium nitride layer 102 formed by bonding the boundaries ofthe island crystal regions to each other becomes thick as shown in FIG.8D, whereby a desired crystal layer is formed on the sapphire substrate100.

[0007]FIGS. 9A to 9D are sectional views, similar to those of FIGS. 8Ato 8D, showing steps of another related art crystal growth method for agallium nitride based compound semiconductor. In this example, ascompared with the example shown in FIGS. 8A to 8D, the crystal growthrate (which is converted into a crystal growth rate in film-like crystalgrowth on a plane) is set to a value being as low as about 1 μm/h.Referring to FIG. 9A, after a low temperature buffer layer is formed ona C-plane 111 of a sapphire substrate 110, like the example shown inFIGS. 8A to 8D, grains each having a size on order of several tens ofnanometers, which are nuclei for formation of a gallium nitride layer112, are formed by stopping the supply of ammonia while continuing thesupply of trimethyl gallium. Referring to FIG. 9B, as the supply oftrimethyl gallium begins again, crystal growth occurs from the grains,to form island crystal regions each laterally extending on the C-plane111. When the crystal growth proceeds at a crystal growth rate of 1 μm/h(which crystal growth rate is a value converted into a crystal growthrate in film-like crystal growth on a plane), boundaries of the islandcrystal regions are bonded to each other as shown in FIG. 9C, andfurther, a gallium nitride layer 112 formed by bonding the boundaries ofthe island crystal regions to each other becomes thick as shown in FIG.9D. In this crystal growth, since the crystal growth rate is lower thanthat in the crystal growth shown in FIGS. 8A to 8D, lateral crystalgrowth becomes predominant growth, with a result that the density ofdislocations becomes smaller than that in the crystal growth shown inFIGS. 8A to 8D.

[0008]FIGS. 10A to 10E are sectional views showing steps of a furtherrelated art crystal growth method for a gallium nitride based compoundsemiconductor, which method is intended to reduce dislocations byselective growth. Referring to FIG. 10A, a gallium nitride layer 122 isformed on a sapphire substrate 120, a silicon oxide film 123 is formedas an anti-surfactant film on the gallium nitride layer 122, and openingportions 124 are formed in the silicon oxide film 123. Referring to FIG.10B, island crystal regions for forming a gallium nitride layer 125 areformed in the opening portions 124 by selective growth. Referring toFIGS. 10C to 10E, as crystal growth proceeds, boundaries of the islandcrystal regions are bonded to each other, whereby a gallium nitridelayer 102 formed by bonding the boundaries of the island crystal regionsto each other is formed to a desired thickness.

[0009] In the above-described technique using a low temperature bufferlayer, as shown in FIGS. 8A to 8D and FIGS. 9A to 9D, since crystalgrowth nuclei formed by the low temperature buffer undergo pseudotwo-dimensional growth, the density of dislocations such as screwdislocations 103 and 113 in lateral growth portions is reduced; however,edge dislocations 104 and 114 occur at portions where boundaries ofisland crystal regions are bonded to each other. As a result, in thecase of using only such a technique, it is believed that a reduction indensity of through-dislocations to a value in a range of less than about10⁹/cm² cannot be obtained. On the other hand, in the technique ofselectively forming a protective film on a first gallium nitride basedcompound semiconductor and re-growing a second gallium nitride basedcompound semiconductor or in the technique of selectively removing afirst gallium nitride based compound semiconductor by RIE or the likeand re-growing a second gallium nitride based compound semiconductor, asshown in FIGS. 10A to 10E, the density of dislocations in lateral growthregions becomes low; however, through-dislocations 126 occur at portionsat which lateral growth regions are bonded to each other, with a resultthat it is difficult to realize a crystal layer having a low density ofdislocations.

[0010] A technique of reducing a density of dislocations by supplying asilicon material as an anti-surfactant at the time of growth of agallium nitride based compound semiconductor film has been known, forexample, in Journal of Crystal Growth 205, 245 (1999). However, even inthis technique, in the step that island crystal regions, each having athree-dimensional shape, undergo pseudo two-dimensional growth, to belaterally grown and thereby bonded to each other, dislocations occur atportions at which boundaries of the island crystal regions are bonded toeach other.

[0011] A method of producing a group III-V compound semiconductor hasbeen disclosed in Japanese Patent Laid-open No. Hei 9-97921, whereinafter a buffer layer is formed, a GaN layer is formed at a growth rateof 1000 Å/min and then a non-doped GaN layer is formed at a growth rateof 200 Å/min. According to this method, a luminous efficiency can beenhanced by forming such a low rate growth layer. This invention isadvantageous in enhancing quality of crystal by forming the low rategrowth layer; however, this invention does not describe a method ofsuppressing through-dislocations, edge dislocations and screwdislocations from a substrate, and therefore, requires reduction indensity of dislocations yet.

[0012] As described above, known crystal growth techniques for a nitridesemiconductor have a limitation in reducing a density of dislocationsinsofar as the technique is singly used, and is disadvantageous in thatthe performance and service life of a semiconductor device formed on asemiconductor layer produced according to known techniques are degraded.

[0013] A need, therefore, exists to develop an improved method forcrystal growth of a nitride semiconductor at a reduced density ofdislocations that can be employed during the manufacture ofsemiconductor devices in a variety of suitable applications.

SUMMARY OF THE INVENTION

[0014] An advantage of the present invention is to provide a crystalgrowth method for a nitride semiconductor, which is capable of reducinga density of dislocations, thereby improving the performance and servicelife of a semiconductor device formed on a semiconductor layer producedaccording to an embodiment of the present invention, and to provide amethod of forming a semiconductor device using the crystal growth methodfor a nitride semiconductor according to an embodiment of the presentinvention.

[0015] In an embodiment, the present invention provides a crystal growthmethod for a nitride semiconductor including a first crystal growth stepof forming a plurality of island crystal regions of a nitridesemiconductor on a base body by vapor-phase growth; and a second crystalgrowth step of further growing the island crystal regions while bondingboundaries of the island crystal regions to each other; wherein acrystal growth rate in the second crystal growth step is higher than acrystal growth rate in the first crystal growth step.

[0016] In the first crystal growth step for forming a plurality ofisland crystal regions of a nitride semiconductor on the base body, theisland crystal regions of the nitride semiconductor can extend in thelateral direction by pseudo two-dimensional growth. In this step, sincethe crystal growth rate is low, the elimination of crystal from a planeparallel to the substrate becomes dominant, with a result that lateralgrowth becomes large. As the island crystal regions extend in thelateral direction, boundaries of the island crystal regions are bondedto each other in any suitable way. At this time, the first crystal stepis shifted or changed to the second crystal step. In this shift, thecrystal growth condition is modulated allowing the crystal growth rateto be increased, preferably rapidly, so as to reduce the elimination ofcrystal from the plane parallel to the substrate. In general, if thegrowth condition is changed, then crystal tends to become stable againstthe changed growth condition. More specifically, if the growth conditionis changed, then the crystal growth direction is changed in accordancewith the changed growth condition. Accordingly, in this case, thecrystal growth direction is changed in accordance with the modulatedcrystal growth condition in such a manner that crystal is grown to buryeach space between an adjacent island crystal regions. As a result,directions of dislocations in the island crystal regions are bent. Thismakes it possible to reduce the density of dislocations caused at thetime of bonding the boundaries of the island crystal regions to eachother.

[0017] In another embodiment, the present invention provides a crystalgrowth method for a nitride semiconductor including a first crystalgrowth step of forming a plurality of island crystal regions of anitride semiconductor on a base body by vapor-phase growth; and a secondcrystal growth step of further growing the island crystal regions whilebonding boundaries of the island crystal regions to each other; whereina crystal growth temperature in the second crystal growth step is lowerthan a crystal growth temperature in the first crystal growth step.

[0018] The modulation of the crystal growth condition due to shifting ofthe first crystal growth step to the second crystal growth step can berealized not only by increasing the crystal growth rate but alsolowering the crystal growth temperature. According to an embodiment ofthe crystal growth method, in the second crystal growth step, thecrystal growth temperature is lowered, to reduce the elimination ofcrystal from the plane parallel to the substrate, thereby changing thecrystal growth direction in the crystal layer, so that crystal is grownto bury each space between adjacent two of the island crystal regions.As a result, directions of dislocations in the island crystal regionsare bent. This makes it possible to reduce the density of dislocationscaused at the time of bonding the boundaries of the island crystalregions to each other.

[0019] In yet another embodiment, the present invention provides acrystal growth method for a nitride semiconductor including a firstcrystal growth step of forming a plurality of island crystal regions ofa nitride semiconductor on a base body by vapor-phase growth; and asecond crystal growth step of further or continued growth of the islandcrystal regions while bonding boundaries of the island crystal regionsto each other; wherein a crystal growth rate in the second crystalgrowth step is higher than a crystal growth rate in the first crystalgrowth step or a crystal growth temperature in the second crystal stepis lower than a crystal growth temperature in the first crystal growthstep; and a state of irregularities on a surface of the base body isobserved, and the first crystal growth step is shifted to the secondcrystal growth step in accordance with the state of irregularities onthe surface of the base body.

[0020] With this configuration, in the second crystal growth step, thecrystal growth condition is modulated by increasing the crystal growthrate and/or lowering the crystal growth temperature, to change thecrystal growth direction in the crystal layer, so that crystal is grownto bury each space between adjacent two of the island crystal regions.As a result, directions of dislocations in the island crystal regionsare bent. This makes it possible to reduce the density of dislocationscaused at the time of bonding the boundaries of the island crystalregions to each other. In addition, according to this crystal growthmethod of this aspect, since the timings at which the boundaries of theisland crystal regions are bonded to each other in the crystal growthstep are detected by observing the state of irregularities on thesurface of the base body, it is possible to efficiently changedirections of dislocations, and hence to further reduce the density ofdislocations caused at the time of bonding the boundaries of the islandcrystal regions to each other.

[0021] In still yet another embodiment, the crystal growth method for anitride semiconductor can be used to form a semiconductor device withdesirable physical properties. The method of forming the semiconductordevice includes a first crystal growth step of forming a plurality ofisland crystal regions of a nitride semiconductor on a base body byvapor-phase growth; a second crystal growth step of further growing theisland crystal regions at a crystal growth rate higher than a crystalgrowth rate in the first crystal growth step while bonding boundaries ofthe island crystal regions to each other; and a semiconductor deviceformation step of forming a semiconductor device on a nitridesemiconductor layer formed by bonding the boundaries of the islandcrystal regions to each other. The semiconductor device can include anysuitable device, such as a semiconductor light emitting diode, asemiconductor laser device, a semiconductor transistor device or thelike.

[0022] With this configuration, in the second crystal growth step, thecrystal growth is performed at a crystal growth rate higher than that inthe first crystal growth step or at a crystal growth temperature lowerthan that in the first crystal growth step, with a result that directionof dislocations in the island crystal regions are bent. This makes itpossible to reduce the density of dislocations caused at the time ofbonding the boundaries of the island crystal regions to each other.Since a semiconductor device is formed on a crystal layer formed at areduced density of dislocations, it is possible to fabricate asemiconductor device having excellent physical properties by the effectof the good crystallinity of the crystal layer.

[0023] Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following detaileddescription of the invention and the Figures.

BRIEF DESCRIPTION OF THE FIGURES

[0024]FIGS. 1A to 1H are sectional views showing steps of an embodimentof a crystal growth method for a nitride semiconductor according to thepresent invention, wherein FIG. 1A shows the step of preparing asapphire substrate, FIG. 1B shows the step of forming a low temperaturebuffer layer, FIG. 1C shows the step of forming grains functioning asnuclei for forming a gallium nitride layer, FIG. 1D shows the step offorming island crystal regions, FIG. 1E shows the step of laterallygrowing the island crystal regions, FIG. 1F shows the step of furthergrowing the island crystal regions while bonding boundaries of theisland crystal regions to each other to form a gallium nitride layer,FIG. 1G shows the step of planarizing the gallium nitride layer, andFIG. 1H shows the step of forming a semiconductor light emitting deviceon the gallium nitride layer.

[0025]FIG. 2 is a graph showing a control for modulating a crystalgrowth rate in an embodiment of the crystal growth method for a nitridesemiconductor according to the present invention.

[0026]FIG. 3 is a graph showing a control for modulating a crystalgrowth temperature in an embodiment of the crystal growth method for anitride semiconductor according to the present invention.

[0027]FIGS. 4A to 4F are sectional views showing steps of an embodimentof the crystal growth method for a nitride semiconductor according tothe present invention, wherein FIG. 4A shows the step of forming a lowtemperature buffer layer, FIG. 4B shows the step of forming a siliconnitride film and forming opening portions in the silicon nitride film,FIG. 4C shows the step of forming selective growth layer portions for agallium nitride layer, FIG. 4D shows the step of laterally growing theselective growth layer portions, FIG. 4E shows the step of furthergrowing the selective growth layer portions while bonding boundaries ofthe selective growth layer portions to each other to form a galliumnitride layer, wherein a void occurs in the bonding portion, and FIG. 4Fshows the step of planarizing the gallium nitride layer;

[0028]FIGS. 5A to 5F are sectional views showing steps of an embodimentof the crystal growth method for a nitride semiconductor according tothe present invention, wherein FIG. 5A shows the step of forming agallium nitride layer, FIG. 5B shows the step of forming a recess in thegallium nitride layer, to form gallium nitride layer portions, FIG. 5Cshows the step of selectively growing the gallium nitride layerportions, FIG. 5D shows the step of laterally growing the galliumnitride layer portions, FIG. 5E shows the step of further growing thegallium nitride layer portions while bonding boundaries of the galliumnitride layer portions to each other to form a gallium nitride layer,wherein a void occurs in the bonding portion, and FIG. 5F shows the stepof planarizing the gallium nitride layer.

[0029]FIGS. 6A to 6E are sectional views showing steps of an embodimentof the crystal growth method for a nitride semiconductor according tothe present invention, wherein FIG. 6A shows the step of forming agallium nitride layer, and then supplying silane gas onto the surface ofthe gallium nitride layer, FIG. 6B shows the step of forming islandcrystal regions, FIG. 6C shows the step of laterally growing the islandcrystal regions, FIG. 6D shows the step of further growing the islandcrystal regions while bonding boundaries of the island crystal regionsto each other to form a gallium nitride layer, and FIG. 6E shows thestep of planarizing the gallium nitride layer.

[0030]FIGS. 7A to 7E are sectional views showing steps of an embodimentof the crystal growth method for a nitride semiconductor according tothe present invention, wherein FIG. 7A shows the step of formingirregularities on a surface of a gallium nitride layer, FIG. 7B showsthe step of forming island crystal regions, FIG. 7C shows the step oflaterally growing the island crystal regions, FIG. 7D shows the step offurther growing the island crystal regions while bonding boundaries ofthe island crystal regions to each other to form a gallium nitridelayer, and FIG. 7E shows the step of planarizing the gallium nitridelayer;

[0031]FIGS. 8A to 8D are sectional views showing a related art crystalgrowth method for a nitride semiconductor, wherein FIG. 8A shows thestep of forming grains as nuclei for forming a gallium nitride layer,FIG. 8B shows the step of forming island crystal regions, FIG. 8C showsthe step of laterally growing the island crystal regions, and FIG. 8Dshows the step of further growing the island crystal regions whilebonding boundaries of the island crystal regions to each other.

[0032]FIG. 9A to 9D are sectional views showing another related artcrystal growth method for a nitride semiconductor, wherein FIG. 9A showsthe step of forming grains as nuclei for forming a gallium nitridelayer, FIG. 9B shows the step of forming island crystal regions, FIG. 9Cshows the step of laterally growing the island crystal regions, and FIG.9D shows the step of further growing the island crystal regions whilebonding boundaries of the island crystal regions to each other.

[0033]FIG. 10A to 10E are sectional views showing a further related artcrystal growth method for a nitride semiconductor, wherein FIG. 10Ashows the step of forming a selective mask, FIG. 10B shows the step offorming island crystal regions, FIG. 10C shows the step of laterallygrowing the island crystal regions, FIG. 10D shows the step of furthergrowing the island crystal regions while bonding boundaries of theisland crystal regions to each other to form a gallium nitride layer,and FIG. 10E shows the step of planarizing the gallium nitride layer.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention generally relates to methods of crystalgrowth on a semiconductor material. More specifically, the presentinvention relates to crystal growth methods for a nitride semiconductor,such as a gallium nitride based semiconductor and a methods for formingsemiconductor devices employing crystal growth methods according to anembodiment of the present invention. The present invention can be usedto make a variety of different and suitable semiconductor devicesincluding, for example, a semiconductor light emitting device, such as asemiconductor light emitting diode, a semiconductor laser, asemiconductor transistor device or the like.

[0035] With respect to the crystal growth methods and the formationmethods for a semiconductor device according to an embodiment of thepresent invention, a technique characterized by using a low temperaturebuffer, a technique characterized by selectively forming ananti-surfactant film, a technique characterized by selectively removingcrystal, a technique characterized by supplying an anti-surfactant, anda technique characterized by forming irregularities each will bedescribed below. In each of these techniques, island crystal regions(including stripe, network, and other shapes), each of which has athree-dimensional shape, undergo pseudo two-dimensional growth, to belaterally grown and thereby bonded to each other, allowing the densityof dislocations in the island crystal regions to be reduced. As usedherein, the term “three-dimensional shape” or like terms means that theisland crystal regions have three-dimensional shapes that can vary inheight therebetween. As used herein, the term “pseudo two-dimensionalgrowth” or the like means lateral growth containing vertical growth. Thepseudo two-dimensional growth is therefore different from“two-dimensional growth” which means only lateral growth withoutvertical growth. According to an embodiment of the present invention, atime during crystal growth at which island crystal regions meet, thatis, boundaries or boundary regions of the island crystal regions arebonded to each other, is not limited to the instant when the islandcrystal regions actually meet, but may be a time around the instant whenthe island crystal regions actually meet insofar as the same effect canbe obtained. The term “base body” or the like used herein may be asubstrate or a typical wafer substrate on which a preferred thin film isformed by crystal growth which may be formed into a plate-shape, may notbe formed into a plate-shape or formed into any suitable shape.

[0036] A crystal growth method for a nitride semiconductor according toan embodiment of the present invention, which is based on a crystalgrowth technique using a low temperature buffer, will be described withreference to FIG. 1A to FIG. 3.

[0037] Referring to FIG. 1A, a sapphire substrate 10 with a C-plane ofsapphire taken as a principal plane of the substrate is placed in anorganometallic growth apparatus (not shown). The sapphire substrate 10is cleaned in a mixed atmosphere (carrier gas) of hydrogen and nitrogenat 1050° C. The temperature is then dropped to 500° C., and at thistemperature, supply of ammonia as a nitrogen material begins, followedby supply of trimethyl gallium as a Ga material, so that as shown inFIG. 1B, an amorphous low temperature buffer layer 11 made from GaN isdeposited to a thickness of about 30 nanometers (“nm”) on the overallprincipal plane of the sapphire substrate 10.

[0038] After the amorphous low temperature buffer layer 11 is deposited,the supply of trimethyl gallium is once stopped while the supply ofammonia is continued, and the temperature is raised up to 1020° C. Underthis condition, as shown in FIG. 1C, the amorphous GaN of the lowtemperature layer 11 is crystallized, to form GaN grains each having asize of several tens nm. The grains function as nuclei for formingdiscontinuous island crystal regions of a gallium nitride layer 12.

[0039] As the supply of trimethyl gallium begins again in the stateshown in FIG. 1E, the grains as nuclei undergo pseudo two-dimensionalgrowth, to form island crystal regions. A crystal growth rate in thiscrystal growth is set at about 1 μm/h. Crystal growth proceeds such thatthe density of nuclei becomes low and the island crystal regions extendin the lateral direction. As shown in FIG. 1D, in this crystal growth,screw dislocations 13 occur in the crystal regions; however, since thecrystal growth rate is as low as about 1 μm/h, the crystal growth in theC-plane direction becomes small and the lateral growth with lessvertical growth becomes predominant, to form the lateral growth portions(island crystal regions) containing less dislocations. On the otherhand, since elimination of crystal from the C-plane is predominant, partof the nuclei formed in the step shown in FIG. 1C is eliminated, with aresult that the density of the nuclei is reduced. In such crystalgrowth, the density of dislocations is reduced, so that the islandcrystal regions for forming a gallium nitride layer 12, which containthrough-dislocations at a density ranging from about 5×10⁸/cm² to about1×10⁹/cm², are obtained.

[0040] Referring to FIG. 1E, at a timing when the island crystal regionsfor forming a gallium nitride layer 12 meet, that is, boundaries 15 ofthe island crystal regions are bonded to each other by pseudotwo-dimensional growth, the above-described first crystal growth step isshifted into a second crystal step. It is to be noted that a timing whenthe boundaries 15 of a pair of island crystal regions are bonded to eachother is not necessarily identical to a timing when the boundaries 15 ofanother pair of island crystal regions. However, according to anembodiment, the timing when the boundaries 15 of the island crystalregions are bonded to each other may be a time around an average timingwhen the boundaries 15 of the island crystal regions are bonded to eachother.

[0041] In the second crystal growth step, the crystal growth rate isincreased. To be more specific, the crystal growth rate, which is set atabout 1 μm/h in the first crystal growth step, is rapidly increased toabout 10 μm/h in the second crystal growth step. Under such a condition,the elimination of crystal from the C-plane becomes smaller than thegrowth of crystal onto the C-plane, so that the island crystal regionsare rapidly grown not only in the lateral direction but also in thevertical direction, and as shown in FIG. 1F, the small island crystalregion is merged in growth of the large island crystal region. As aresult, dislocations occurring in the small crystal region are bent to abonding portion 16 at which the small crystal region is bonded to thelarge crystal region, or are terminated at another dislocation having asuitable Burgers vector. The density of the remaining dislocations inthe small crystal region merged with the large crystal region is thusreduced. Then, as the crystal growth proceeds at a usual crystal growthrate of about 4 μm/h, a gallium nitride layer 12 having a good surfacemorphology and containing through-dislocation at a reduced density isformed (see FIG. 1G). As a result of experiments performed by thepresent inventors, it is found to obtain a gallium nitride layer inwhich the density of through-dislocations is reduced to about 2×10⁸/cm²to about 5×10⁸/cm².

[0042] The gallium nitride layer 12, in which the density ofthrough-dislocations is reduced by rapidly increasing the crystal growthrate, can be used as an underlying portion of a semiconductor device. Asshown in FIG. 1H, an n-type gallium nitride layer 17 doped with silicon,an active layer 18 made from InGaN, and a p-type gallium nitride layer19 doped with magnesium are stacked on the gallium nitride layer 12; theactive layer 18 and the p-type gallium nitride layer 19 are selectivelyremoved to expose part of the n-type gallium nitride layer 17; and ann-side electrode 8 and a p-side electrode 9 are formed on the n-typegallium nitride layer 17 and the p-type gallium nitride layer 19 byvapor-deposition or the like, respectively, to form a semiconductorlight emitting diode device. Since the density of through-dislocationsin the gallium nitride layer 12 used as the underlying layer is reduced,characteristics, particularly, a light emission characteristic of thesemiconductor light emitting diode device can be enhanced. According toan embodiment, the semiconductor light emitting diode device made from agallium nitride based compound is formed on the gallium nitride layer12. It should be appreciated that any suitable device can be formed onthe gallium nitride layer 12 including, for example, a semiconductorlaser device, an electric field effect transistor, or another activedevice. The device formed on the gallium nitride layer 12 is not limitedto a planar type device but may be a semiconductor light emitting diodedevice having a pyramid structure, such as a hexagonal pyramid structurein which an active layer is formed by making use of facets, such asS-planes.

[0043] In the crystal growth method for a nitride semiconductoraccording to an embodiment, at the timing when the boundaries 15 of theisland crystal regions are bonded to each other, the crystal growth rateis largely increased from about 1 μm/h to about 10 μm/h, so thatdislocations in a small crystal region are bent to a bonding portion atwhich the small crystal region is bonded to the large island crystalregion, or are terminated at another dislocation having a suitableBurgers vector, with a result that the density of the remainingdislocations in the small island crystal region is reduced. Accordingly,the density of dislocations in the entire crystal can be largelyreduced. As a result, in the case of forming a semiconductor device suchas a semiconductor light emitting diode device on the gallium nitridelayer thus produced, characteristics of the device can be significantlyimproved.

[0044] According to the above-described related art crystal growthmethod shown in FIGS. 9A to 9D, after the temperature is raised to 1020°C. crystal growth proceeds at a crystal growth rate of 1 μm/h, so thatit is possible to obtain an effect of reducing the density of nuclei,thereby lowering the density of through-dislocations. However, since theelimination of crystal from the C-plane is large, there arises a problemthat the surface morphology is degraded. According to an embodiment ofthe present invention, in the first crystal growth step, since thecrystal growth rate is low, it is possible to obtain an effect ofreducing the density of nuclei, thereby lowering the density ofdislocations, and in the second crystal growth step, since the crystalgrowth rate is high, it is possible to suppress the elimination ofcrystal from the C-plane to some extent and hence to form a crystallayer having a good surface morphology.

[0045]FIG. 2 is a graph showing a change in crystal growth rate withelapsed time upon crystal growth performed by controlling the crystalgrowth rate. In the figure, an island bonding timing Ic is the time whenthe boundaries of the island crystal regions are bonded to each other.In the first crystal growth step, crystal growth proceeds at a lowcrystal growth rate of about 1 μm/h, and at a time around the islandbonding timing Ic, the crystal growth is shifted to crystal growth at ahigh crystal growth rate of, for example, about 10 μm/h. According to anembodiment, after the boundaries of the island crystal regions arebonded to each other, the crystal growth rate is changed from about 10μm/h to about 4 μm/h. In the graph, the change in crystal growth rate isrepresented in stepped segments. It should be appreciated that thechange in the crystal growth rate may be controlled in any suitablemanner including, for example, a change represented in a continuouscurve linear manner.

[0046] In an embodiment, to realize the effect of reducing the densityof dislocations, the crystal growth rate in the second crystal growthstep is required to be about twice or more than the crystal growth ratein the first crystal growth step. If the crystal growth rate in thesecond crystal growth step is less than about twice the crystal growthrate in the first crystal growth step, then the difference in crystalgrowth condition between the first and second crystal growth steps istoo small to achieve the instantaneous increase in crystal growth rateand the bending of dislocations. The crystal growth rate in the secondcrystal growth step is preferably set to be as large as about five timesor more than the crystal growth rate in the first crystal growth step.In the first crystal growth step, the crystal growth rate upon lateralgrowth is required to be set at about 3 μm/h or less. If more than about3 μm/h, it is difficult to realize the condition under which eliminationof crystal becomes predominant. The crystal growth rate upon lateralgrowth is preferably set at about 1.5 μm/h or less. It is to be notedthat the crystal growth rate upon growth of the island crystal regionscannot be simply defined, and therefore, according to the presentinvention, the crystal growth rate upon growth of the island crystalregions, which is expressed by a volume-increasing rate, is based on acrystal growth rate upon film-like growth on a plane.

[0047] In an embodiment, the crystal growth rate in the second crystalgrowth step is required to be set at about 2 μm/h or more in order toachieve the condition under which elimination of crystal becomes smallat the time when the island crystal regions meet. If less than about 2μm/h, the crystal growth condition cannot be changed from the conditionunder which elimination of crystal is predominant to the condition underwhich elimination of crystal is small. As a result, it fails to achievethe instantaneous increase in crystal growth rate and the bending ofdislocations. The crystal growth rate in the second crystal growth stepis preferably set at about 4 μm/h or more.

[0048] In the case of changing the crystal growth rate between the firstand second crystal growth steps, it is not required to change thecrystal growth temperature between the first and second crystal growthsteps. The selection of the same crystal growth temperature between thefirst and second crystal growth steps is effective to simplify thecrystal growth process. If the crystal growth rate and the crystalgrowth temperature are simultaneously changed between the first andsecond crystal growth steps, then the repeatability of the process maybe degraded. From this viewpoint, in the case of changing the crystalgrowth rate between the first and second crystal growth steps, it ispreferable not to change the crystal growth temperature between thefirst and second crystal growth steps.

[0049] In the case of shifting the first crystal growth step to thesecond crystal growth step, according to an embodiment, the crystalgrowth rate is changed from a low growth rate to a high growth rate;however, the same effect of reducing through-dislocations can beobtained by changing the crystal growth temperature from a hightemperature to a low temperature. To be more specific, in the case ofchanging the crystal growth temperature into a low temperature of, forexample, 980° C., elimination of crystal atoms from the growth plane isreduced, so that dislocations occurring in the vicinity of a portion atwhich the boundary of a small island crystal region is bonded to theboundary of another small island crystal region are bent to a portion atwhich the boundary of the small crystal region is bonded to the boundaryof another large crystal region, or are terminated at anotherdislocation having a suitable Burgers vector, with a result that thedensity of the remaining dislocations is reduced.

[0050]FIG. 3 is a graph showing an example in which the crystal growthtemperature is changed from a high temperature to a low temperature atthe time of shifting the first crystal growth step to the second crystalgrowth step. Upon formation of the low temperature buffer layer, thecrystal growth temperature is set at about 500° C., and at the firstcrystal growth step, high temperature crystal growth proceeds at thecrystal growth rate of about 3 μm/h and at a crystal growth temperatureof about 1050° C. Next, at the timing of the island bonding Ic at whichthe boundaries of the island crystal regions are bonded to each other,the high temperature crystal growth is shifted to low temperaturecrystal growth which is performed at about 980° C. With this lowtemperature crystal growth at about 980° C., dislocations in thevicinity of each bonding portion are largely bent. After the islandcrystal regions meet, the crystal growth is continued at about 1020° C.As a result, a crystal film with a reduced density of dislocations and agood surface morphology can be obtained.

[0051] In the first crystal growth step, unless the crystal growthtemperature is somewhat high, the condition in which elimination ofcrystal is predominant cannot be realized even by lowering the crystalgrowth rate. Accordingly, in the case of growing a gallium nitridelayer, the minimum temperature for obtaining the condition in whichelimination of crystal is predominant may be set at about 980° C.,preferably about 1000° C. or more. A difference in temperature betweenthe high temperature growth and the low temperature growth is requiredto be about 20° C. or more, preferably about 40° C. or more. If thedifference is less than about 20° C., the difference in crystal growthcondition between the high temperature growth and the low temperaturegrowth is excessively small, so that it fails to realize theinstantaneous increase in crystal growth rate and the bending ofdislocations. Further, unless the crystal growth temperature at the timeof lateral growth is set at 980° C. or more, particularly about 1000° C.or more, the condition in which elimination of crystal is predominantcannot be realized. The crystal growth temperature for obtaining thecondition in which elimination of crystal is small in the second crystalgrowth step is required to be set at about 1050° C. or less, preferablyabout 1020° C. or less.

[0052] To modulate the crystal growth rate or the crystal growthtemperature at a timing when boundaries of island crystal regions arebonded to each other as described in this embodiment, there can be useda method of optically observing a surface roughness of the crystalgrowth layer and determining the timing when boundaries of islandcrystal regions are bonded to each other in accordance with the observedsurface roughness of the crystal growth layer. For example, it iseffective that in the method shown in FIGS. 1A to 1H, a time when thesurface roughness of the crystal growth layer during crystal growth ismeasured, and the crystal growth rate or the crystal growth temperatureis rapidly increased on the based of the measured time. As the opticalmeasurement method, there may be used a method of observing lightreflected from the surface of a crystal growth layer, or a method ofobserving the transmission of light emitted from a susceptor or the likethrough a wafer.

[0053] In an embodiment, the crystal growth rate is modulated at thetime of shifting the first crystal growth step to the second crystalgrowth step, and in the modification, the crystal growth temperature ismodulated at the time of shifting the first crystal growth step to thesecond crystal growth step; however, the control of the crystal growthcondition may be performed in such a manner as to modulate both thecrystal growth rate and the crystal growth temperature at the time ofshifting the first crystal growth step to the second crystal growthstep. While the compound semiconductor layer is exemplified by a galliumnitride layer in this embodiment, it may be another wurtzite typecompound semiconductor layer in consideration of the fact that a facetstructure will be formed in the step subsequent to the step of producingthe compound semiconductor layer. The same effect can be obtained evenby using another wurtzite type compound semiconductor layer. Apreferable compound semiconductor layer is a layer made from a nitridesemiconductor having a wurtzite type crystal structure, a BeMgZnCdSbased compound semiconductor, or a BeMgZnCdO based compoundsemiconductor.

[0054] Specific examples of the above nitride semiconductors used forforming the crystal layer according to this embodiment may include agroup III based compound semiconductor, a gallium nitride (GaN) basedcompound semiconductor, an aluminum nitride (AIN) based compoundsemiconductor, an indium nitride (InN) based compound semiconductor, anindium gallium nitride (InGaN) based compound semiconductor, an aluminumgallium nitride (AlGaN) based compound semiconductor and other suitablesemiconductor materials. In particular, a gallium nitride based compoundsemiconductor is preferably used. It is to be noted that according tothe present invention, InGaN, AlGaN or the like does not necessarilymean a nitride semiconductor having only a ternary mixed crystalstructure, and similarly, GaN or the like does not necessarily mean anitride semiconductor having only a binary mixed crystal structure. Forexample, even if InGaN contains a trace of Al and inevitable impuritiesin a range not to change the function of InGaN, such a material can beused for forming the crystal growth layer according to the presentinvention.

[0055] A crystal growth method for a nitride gallium based nitridesemiconductor using a selective growth mask according to an embodimentof the present invention, which is based on a technique of reducing thedensity of dislocations by modulation of a crystal growth rate inaddition to an effect of reducing the density of dislocations by usingthe selective growth mask, will be described with reference to FIGS. 4Ato 4F.

[0056] Referring to FIG. 4A, a gallium nitride layer 21 is grown to athickness of about 2 μm on a sapphire substrate 20 with a C-plane ofsapphire taken as a principal plane of the substrate by using a lowtemperature buffer technique in accordance with an organometallicvapor-phase growth process or other suitable process. Ammonia is used asa nitrogen material, trimethyl gallium is used as a gallium material,and hydrogen and nitrogen are used as a carrier gas. The gallium nitridelayer 21 thus formed contains through-dislocations 22 at a density ofabout 1×10⁹/cm² to about 2×10⁹/cm².

[0057] Referring to FIG. 4B, a silicon oxide film 23 as ananti-surfactant film is formed on the gallium nitride layer 21, andstripe-shaped opening portions 24 extending in a {1, −1, 0, 0} directionof the gallium nitride layer 21 are formed in the silicon oxide film 23by photolithography. The thickness of the silicon oxide film 23 istypically set at about 200 nm. The opening portions 24, each having awidth of about 3 μm, are spaced from each other at intervals of about 10μm. Each of the width and interval of the opening portions 24 is notlimited to the value shown in this embodiment but may be set at anyother value.

[0058] After the opening portions 24 are formed in the silicon nitridefilm 23, the sapphire substrate 20 is put in the organometallicvapor-phase growth apparatus again, and the temperature is raised toabout 1020° C. while hydrogen, nitrogen and ammonia are made to flow andthen the supply of trimethyl gallium begins, so that as shown in FIG.4C, a selective growth layer portion 25 made from gallium nitride, whichis formed into a truncated hexagonal pyramid shape in cross-section, isformed from each of the opening portions 24.

[0059] At this time, the supplied amount of trimethyl gallium is set ata value allowing a crystal growth rate (which is expressed by a valuebased on that at the time of film-like crystal growth) of about 4 μm/h.Selective growth can be continued at such a growth rate; however, whenthe growth rate is reduced by about ¼, elimination of crystal from theC-plane becomes predominant, and thereby lateral growth becomespredominant. This is effective to make a thickness of the growth layerthin and to suppress a camber of the growth layer. Consequently, asshown in FIG. 4D, the selective growth layer portions 25 are grown onthe silicon oxide film 23 in the lateral and vertical directions, sothat boundaries 26 of the selective growth layer portions 25 grown fromthe adjacent opening portions 24 are bonded to each other, to form abonding portion 27.

[0060] At a timing when the bonding portion 27 is formed or slightlybefore or after the timing, the crystal growth rate is rapidly increasedto about 20 μm/h. At such a crystal growth rate in the second crystalgrowth step, the crystal growth at the C-plane is accelerated. That isto say, the crystal is instantaneously grown in the vertical directionand is then quickly grown in the lateral and vertical directions whilekeeping the analogous shape. Accordingly, since the material is notsupplied to the bonding portion 27, a void 28 with no crystal remains inthe bonding portion 27 as shown in FIG. 4E. Dislocations 22 a bent fromthe opening portions 24 and propagating to the bonding portion 27 areterminated at the void 28. After that, as shown in FIG. 4F, the crystalgrowth proceeds at a crystal grow rate returned to about 4 μm/h. As aresult of experiments performed by the present inventors, it is revealedthat the density of dislocations at a central portion of the mask isreduced to about half of the density of dislocations in the related artmethod shown in FIGS. 10A to 10E, and that a thin growth film in which adensity of dislocations is low can be obtained by the method accordingto an embodiment of the present invention.

[0061] According to the crystal growth method for a nitridesemiconductor according to an embodiment, by rapidly increasing thecrystal growth rate to about 20 μm/h in the second crystal growth step,dislocations can be terminated at the void 28. Accordingly, dislocationsin crystal can be further reduced by combination with the effect ofreducing dislocations by selective growth. It is to be noted thataccording to an embodiment, the crystal growth rate is increased at thetime of shifting the first crystal growth step to the second crystalgrowth step; however, as previously discussed, the crystal growthtemperature may be lowered at the time of shifting the first crystalgrowth step to the second crystal growth step. To modulate the crystalgrowth rate or the crystal growth temperature at a timing whenboundaries of the island crystal regions are bonded to each other, therecan be used the above-described technique of optically observing asurface roughness of the nitride compound, thereby determining thetiming when boundaries of the island crystal regions are bonded to eachother in accordance with the observed surface roughness of the nitridecompound.

[0062] In an embodiment, the crystal growth method can be applied toproduction of a semiconductor device as previously discussed. Thesemiconductor device can include any suitable device, such as asemiconductor light emitting diode device, a semiconductor laser devicemade from a gallium nitride based compound, and another semiconductordevice such as an electric field effect transistor, and further otheractive devices. It should be appreciated that a device to be formed on agallium nitride layer made pursuant to an embodiment of the presentinvention is not limited to a planar type device but may be asemiconductor light emitting diode device having a pyramid structuresuch as a hexagonal pyramid structure in which an active layer is formedby making use of a facet such as S-planes. While the compoundsemiconductor layer is preferably a gallium nitride layer in anembodiment, it may be another wurtzite type compound semiconductor layerin consideration of the fact that a facet structure will be formed inthe step subsequent to the step of producing the compound semiconductorlayer. The same effect can be obtained even by using another wurtzitetype compound semiconductor layer. A preferable compound semiconductorlayer is a layer made from a nitride semiconductor having a wurtzitetype crystal structure, a BeMgZnCdS based compound semiconductor, or aBeMgZnCdO based compound semiconductor.

[0063] In an embodiment, a crystal growth method for a nitridesemiconductor, which is based on a technique of selectively removing agallium nitride layer and laterally growing the remaining crystal,thereby reducing the density of dislocations, will be described withreference to FIGS. 5A to 5F.

[0064] Referring to FIG. 5A, a gallium nitride layer 31 is grown to athickness of about 1 μm on a sapphire substrate 30 with a C-plane ofsapphire taken as a principal plane of the substrate by using a lowtemperature buffer layer in accordance with the organometallicvapor-phase growth process. At this time, the gallium nitride layer 31contains through-dislocations 32.

[0065] Referring to FIG. 5B, the gallium nitride layer 31 are partiallyremoved by photolithography and RIE in such a manner that stripe shapedportions of the gallium nitride layer 31 remain. The stripe shapedportions, each having a width of about 3 μm, are spaced from each otherat intervals of about 10 μm. In this case, the etching is performeduntil the surface of the sapphire substrate 30 is slightly removed. Thesapphire substrate 30 is put in the organometallic vapor-phase growthapparatus again, and the temperature is raised to about 1020° C. whilehydrogen, nitrogen and ammonia are made to flow and then the supply oftrimethyl gallium begins to start crystal growth. In accordance with therelated art method, crystal growth proceeds at a constant crystal growthrate (which is a value converted into a crystal growth rate at the timeof film-like growth) of about 4 μm/h. On the contrary, according to thisembodiment of the present invention, crystal growth can start at acrystal growth rate of about 1 μm/h. As shown in FIGS. 5C and 5D, eachof the rectangular gallium nitride layer portions 31 is laterally grownwith formation of a stable plane, (1, 1, −2, 2) plane. In this stage, aspreviously discussed, since the crystal growth rate is low, lateralgrowth is predominant, so that a recess 33 remain in the surface of thesapphire substrate 30.

[0066] At a timing when boundaries 34 of the gallium nitride layerportions 31 laterally grown are bonded to each other to form a bondingportion 35 or slightly before and after the timing, the crystal growthrate is rapidly increased to about 20 μm/h. At such a crystal growthrate in the second crystal growth step, the crystal growth at theC-plane is accelerated. That is to say, the crystal is instantaneouslygrown in the vertical direction and is then quickly grown in the lateraland vertical directions while keeping the analogous shape. As a result,since the material is not supplied to the bonding portion 35, a void 36with no crystal remains in the bonding portion 35 as shown in FIG. 5E.Dislocations 32 a bent in the gallium nitride portions 31 andpropagating to the bonding portion 35 are terminated at the void 36.Next, as shown in FIG. 5F, the crystal growth proceeds at a crystal growrate returned to about 4 μm/h. After that, in the case of forming asemiconductor light emitting diode as a semiconductor device, an n-typegallium nitride layer, an active layer, and a p-type gallium nitridelayer are sequentially stacked on the crystal growth layer, andrespective electrodes are formed, to thereby form the semiconductorlight emitting diode.

[0067] As previously discussed, by rapidly increasing the crystal growthrate to 20 μm/h in the second crystal growth step, dislocations can beterminated at the void 36 according to an embodiment of the presentinvention. As a result, dislocations in crystal can be further reducedby combination with the effect of reducing dislocations by selectivegrowth. It is to be noted that according to an embodiment, the crystalgrowth rate is increased at the time of shifting the first crystalgrowth step to the second crystal growth step; however, as previouslydiscussed, the crystal growth temperature may be lowered at the time ofshifting the first crystal growth step to the second crystal growthstep. To modulate the crystal growth rate or the crystal growthtemperature at a timing when boundaries of the island crystal regionsare bonded to each other, there can be used the above-described means ofoptically observing a surface roughness of the nitride compound, therebydetermining the timing when boundaries of the island crystal regions arebonded to each other in accordance with the observed surface roughnessof the nitride compound.

[0068] As previously discussed, the crystal growth method according toan embodiment can be applied to production of a semiconductor devicesuch as a semiconductor light emitting diode device or a semiconductorlaser device made from a gallium nitride based compound, and anothersemiconductor device such as an electric field effect transistor, andfurther other active devices. A device to be formed on a gallium nitridelayer produced according to this embodiment is not limited to a planartype device but may be a semiconductor light emitting diode devicehaving a pyramid structure such as a hexagonal pyramid structure inwhich an active layer is formed by making use of a facet such asS-planes. While the compound semiconductor layer is exemplified by agallium nitride layer in this embodiment, it may be another wurtzitetype compound semiconductor layer in consideration of the fact that afacet structure will be formed in the step subsequent to the step ofproducing the compound semiconductor layer. The same effect can beobtained even by using another wurtzite type compound semiconductorlayer. A preferable compound semiconductor layer is a layer made from anitride semiconductor having a wurtzite type crystal structure, aBeMgZnCdS based compound semiconductor, or a BeMgZnCdO based compoundsemiconductor.

[0069] In an embodiment, a crystal growth method for a nitridesemiconductor according to this embodiment, which is based on atechnique of supplying an anti-surfactant on a surface of a nitridegallium based compound semiconductor layer, forming island crystalregions, and laterally growing the island crystal regions, therebyreducing the density of dislocations, will be described with referenceto FIGS. 6A to 6E.

[0070] Referring to FIG. 6A, a gallium nitride layer 41 is grown to athickness of 1 μm on a sapphire substrate 40 with a C-plane taken as aprincipal plane of the substrate by using a low temperature buffer layerin accordance with the organometallic vapor-phase growth process. Inthis stage, the gallium nitride layer 41 contains screw dislocations 43and edge dislocations 42.

[0071] The supply of trimethyl gallium is stopped and silane gas as asilicon material is supplied for about 5 minutes. Silicon functions asan anti-surfactant, so that crystal growth is inhibited at portions,covered with silicon, of the surface of the gallium nitride layer 41.When the supply of trimethyl gallium at a supplied amount allowing acrystal growth rate (which is a value converted into a crystal growthrate at the time of film-like crystal growth) of about 4 μm/h beginsagain, crystal growth starts from portions, exposed through pin-holesnot covered with silicon, of the gallium nitride layer 41, so thatisland crystal regions 44 are formed on the surface of the galliumnitride layer 41 as shown in FIG. 6B. According to the related artmethod, the crystal growth proceeds at the constant crystal growth rateof about 4 μm/h. According to an embodiment of the present invention,when the island crystal regions 44 are formed, the crystal growth rateis changed to about 1 μm/h. Subsequently, as shown in FIGS. 6C and 6D,at a timing when boundaries 45 of the laterally grown island crystalregions 44 are bonded to each other to form a bonding portion 46, thecrystal growth rate is rapidly increased to about 10 μm/h. With thisrapid increase in crystal growth rate, dislocations in the vicinity ofthe bonding portion 46 are bent, whereby the density of dislocations isreduced.

[0072] After that, the crystal growth proceeds at the crystal growthrate returned to about 4 μm/h, whereby the growth layer is planarized asshown in FIG. 6E. As previously discussed, since lateral growth becomespredominant in the first crystal growth step in which the crystal growthrate is low, the thickness of the growth layer can be made thin, andsince the crystal growth rate is rapidly increased at the time ofshifting the first crystal growth step to the second crystal growthstep, the density of dislocations at the bonding portion can be reducedas compared with the related art method. In an embodiment, the crystalgrowth rate is increased at the time of shifting the first crystalgrowth step to the second crystal growth step; however, as previouslydiscussed, the crystal growth temperature may be lowered at the time ofshifting the first crystal growth step to the second crystal growthstep. To modulate the crystal growth rate or the crystal growthtemperature at a timing when boundaries of the island crystal regionsare bonded to each other, there can be used the above-described means ofoptically observing a surface roughness of the nitride compound, therebydetermining the timing when boundaries of the island crystal regions arebonded to each other in accordance with the observed surface roughnessof the nitride compound.

[0073] As previously discussed, the crystal growth method according toan embodiment can be applied to production of a semiconductor devicesuch as a semiconductor light emitting diode device or a semiconductorlaser device made from a gallium nitride based compound, and anothersemiconductor device such as an electric field effect transistor, andfurther other active devices. A device to be formed on a gallium nitridelayer produced according to this embodiment is not limited to a planartype device but may be a semiconductor light emitting diode devicehaving a pyramid structure such as a hexagonal pyramid structure inwhich an active layer is formed by making use of a facet such asS-planes. While the compound semiconductor layer is exemplified by agallium nitride layer in this embodiment, it may be another wurtzitetype compound semiconductor layer in consideration of the fact that afacet structure will be formed in the step subsequent to the step ofproducing the compound semiconductor layer.

[0074] In an embodiment, a crystal growth method for a nitridesemiconductor according to this embodiment, which is based on atechnique of forming irregularities on a surface of a gallium nitridelayer, forming island crystal regions, and laterally growing the islandcrystal regions, thereby reducing the density of dislocations, will bedescribed with reference to FIGS. 7A to 7E.

[0075] Referring to FIG. 7A, a gallium nitride layer 51 is grown to athickness of about 1 μm on a sapphire substrate 50 with a C-plane takenas a principal plane of the substrate by using a low temperature bufferlayer in accordance with the organometallic vapor-phase growth process.Subsequently, irregularities are formed on the surface of the galliumnitride layer 51 by etching or the like. In this stage, the galliumnitride layer 51 contains screw dislocations 53 and edge dislocations52.

[0076] The supply of trimethyl gallium begins, so that island crystalregions 54 are formed on the surface of the gallium nitride layer 51 asshown in FIG. 7B. Since part of the screw dislocations 53 and the edgedislocations 52 are terminated at the irregularities on the surface ofthe gallium nitride layer 51, the density of the dislocations isreduced. In an embodiment, when the island crystal regions 54 areformed, the crystal growth rate is changed to about 1 μm/h. Then, asshown in FIGS. 7C and 7D, at a timing when boundaries 55 of thelaterally grown island crystal regions 54 are bonded to each other toform a bonding portion 56, the crystal growth rate is rapidly increasedto about 10 μm/h. With this rapid increase in crystal growth rate,dislocations in the vicinity of the bonding portion 56 are bent, wherebythe density of dislocations is further reduced.

[0077] After that, the crystal growth proceeds at the crystal growthrate returned to about 4 μm/h, whereby the growth layer is planarized asshown in FIG. 7E. As previously discussed, since lateral growth becomespredominant in the first crystal growth step in which the crystal growthrate is low, the thickness of the growth layer can be made thin, andsince the crystal growth rate is rapidly increased at the time ofshifting the first crystal growth step to the second crystal growthstep, the density of dislocations at the bonding portion can be reducedas compared with the related art method. In an embodiment, the crystalgrowth rate is increased at the time of shifting the first crystalgrowth step to the second crystal growth step; however, as previouslydiscussed, the crystal growth temperature may be lowered at the time ofshifting the first crystal growth step to the second crystal growthstep. To modulate the crystal growth rate or the crystal growthtemperature at a timing when boundaries of the island crystal regionsare bonded to each other, there can be used the above-described means ofoptically observing a surface roughness of the nitride compound, therebydetermining the timing when boundaries of the island crystal regions arebonded to each other in accordance with the observed surface roughnessof the nitride compound.

[0078] As previously discussed, the crystal growth method according toan embodiment can be applied to production of a semiconductor devicesuch as a semiconductor light emitting diode device or a semiconductorlaser device made from a gallium nitride based compound, and anothersemiconductor device such as an electric field effect transistor, andfurther other active devices. A device to be formed on a gallium nitridelayer produced according to an embodiment of the present invention isnot limited to a planar type device but may be a semiconductor lightemitting diode device having a pyramid structure such as a hexagonalpyramid structure in which an active layer is formed by making use of afacet such as S-planes. While the compound semiconductor layer isexemplified by a gallium nitride layer in this embodiment, it may beanother wurtzite type compound semiconductor layer in consideration ofthe fact that a facet structure will be formed in the step subsequent tothe step of producing the compound semiconductor layer.

[0079] As described above, in the crystal growth method for a nitridesemiconductor according to an embodiment of the present invention, acrystal growth rate or a crystal growth temperature is modulated at thetime of shifting a first crystal growth step to a second crystal growthstep. As a result, dislocations at a bonding portion at which a smallisland crystal region is bonded to another small island crystal regionare bent to a bonding portion at which the small island crystal regionis bonded to another large island crystal region, or terminated atanother dislocation or a void, with a result that the density ofdislocations is reduced. Further, by combination with a related art anyother dislocation reducing technique, dislocations in the vicinity of aboundary of an island crystal region and through-dislocations can bereduced.

[0080] In the method of forming a semiconductor device according to anembodiment of the present invention, since a crystal layer in which thedensity of dislocations in crystal is reduced by the above-describedcrystal growth method can be used for forming a semiconductor device, itis believed that characteristics of the semiconductor device can besignificantly improved.

[0081] It should be understood that various changes and modifications tothe presently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is, therefore, intendedthat such changes and modifications be covered by the appended claims.

The invention is claimed as follows:
 1. A method of crystal growth for anitride semiconductor, the method comprising the steps of: forming aplurality of island crystal regions each defining a boundary region on abase body of the nitride semiconductor using vapor-phase growth at afirst crystal growth rate; and continuing growth of the island crystalregions at a second crystal growth rate that is greater than the firstcrystal growth rate while bonding occurs between the boundary portionsof the island crystal regions.
 2. The method according to claim 1,wherein the second crystal growth rate is two times or more greater thanthe first crystal growth rate.
 3. The method according to claim 1,wherein the first crystal growth rate is about 3 μm/hour or less basedon a crystal growth rate associated with film crystal growth on a plane.4. The method according to claim 1, wherein the second crystal growthrate is about 2 μm/hour or more based on a crystal growth rateassociated with film crystal growth on a plane.
 5. The method accordingto claim 1, wherein the crystal growth at the first crystal growth rateis performed at a temperature of about 980° C. or more.
 6. The methodaccording to claim 1, wherein the second crystal growth rate isperformed at a second crystal growth temperature substantially equal toa temperature at which the crystal growth at the first crystal growthrate is performed.
 7. The method according to claim 1 further comprisingthe step of forming a low temperature buffer layer on the base bodyprior to crystal growth.
 8. The method according to claim 1 furthercomprising the step of selectively forming an anti-surfactant film onthe base body prior to crystal growth.
 9. The method according to claim1 further comprising the step of supplying an anti-surfactant on thebase body prior to crystal growth.
 10. The method according to claim 1further comprising the step of selectively removing at least a portionof a surface region of the base body prior to crystal growth.
 11. Themethod according to claim 1, wherein irregularities are formed on atleast a portion of a surface of the base body prior to crystal growth.12. The method according to claim 1, wherein the island crystal regionseach include a three-dimensional shape that is laterally grown due to apseudo two-dimensional growth.
 13. A method of crystal growth for anitride semiconductor, the method comprising the steps of: forming aplurality of island crystal regions on a base body of the nitridesemiconductor using vapor-phase growth wherein the island crystalregions each include a boundary region; and continuing growth of theisland crystal regions while bonding of the boundary regions of theisland crystal regions occurs wherein the island crystal regions aregrown at a crystal growth rate that is increased at a time when bondingof the boundary regions approximately begins.
 14. A method of crystalgrowth for a nitride semiconductor, the method comprising the steps of:forming a plurality of island crystal regions on a base body of anitride semiconductor using vapor-phase growth at a first crystal growthtemperature wherein the island crystal regions each include a boundaryregion; and continuing growth of the island crystal regions at a secondcrystal growth temperature occurs while bonding of the boundary regionsof the island crystal regions wherein the second crystal growthtemperature is lower than first crystal growth temperature.
 15. Themethod according to claim 14, wherein the first crystal growthtemperature is about 20° C. or higher than the second crystal growthtemperature.
 16. The method according to claim 14, wherein the firstcrystal growth temperature is about 40° C. or higher than the secondcrystal growth temperature.
 17. The method according to claim 14,wherein the first crystal growth temperature is about 980° C. or more.18. The method according to claim 14, wherein the second crystal growthtemperature is about 1050° C. or less.
 19. The method according to claim14, wherein the step of forming the island crystal regions is conductedat a first crystal growth rate ranging from about 3 μm/hour or lessbased on a crystal growth rate in film crystal growth on a plane. 20.The method according to claim 19, wherein the step of continuing growthof the island crystal regions is performed at a crystal growth ratesubstantially equal to the first crystal growth rate.
 21. The methodaccording to claim 14 further comprising the step of forming a lowtemperature buffer layer on the base body prior to crystal growth. 22.The method according to claim 14 further comprising the step ofselectively forming an anti-surfactant film on the base body prior tocrystal growth.
 23. The method according to claim 14 further comprisingthe step of applying an anti-surfactant to the base body prior tocrystal growth.
 24. The method according to claim 14 further comprisingthe step of selectively removing a surface portion of the base bodyprior to crystal growth.
 25. The method according to claim 14, whereinirregularities are formed on a surface portion of the base body prior tocrystal growth.
 26. The method according to claim 14, wherein the islandcrystal regions each include a three-dimensional shape that can belaterally grown due to a pseudo two-dimensional crystal growth.
 27. Amethod of crystal growth for a nitride semiconductor, the methodcomprising the steps of: forming a plurality of island crystal regionson a base body of the nitride semiconductor using vapor-phase growthduring a first crystal growth phase including a first crystal growthrate and a first crystal growth temperature wherein the island crystalregions each include a boundary region; continuing growth of the islandcrystal regions while bonding of the boundary regions during a secondcrystal growth phase including a second crystal growth phase rate and asecond crystal growth temperature wherein at least one of the secondcrystal growth rate is greater than the first crystal growth rate andthe second crystal growth temperature is lower than the first crystalgrowth temperature; and changing from the first crystal growth phase tothe second crystal growth phase once a state of irregularities on asurface portion of the base body is observed.
 28. A method ofmanufacturing a semiconductor device, the method comprising the stepsof: forming a plurality of island crystal regions on a base body of anitride semiconductor using vapor-phase growth at a first crystal growthrate wherein the island crystal regions each include a boundary region;continuing growth of the island crystal regions at a second crystalgrowth rate greater than the first crystal growth rate while bonding ofthe boundary regions occurs; and forming the semiconductor device on alayer of the nitride semiconductor formed by bonding the boundaryregions of the island crystal regions.
 29. The method according to claim28, wherein the semiconductor device is selected from the groupconsisting of a semiconductor light emitting device and a semiconductortransistor device.
 30. The method according to claim 29, wherein thesemiconductor light emitting device is selected from the groupconsisting of a semiconductor light emitting diode and a semiconductorlaser device.
 31. A method of forming a semiconductor device, the methodcomprising the steps of: forming a plurality of island crystal regionson a base body of a nitride semiconductor using vapor-phase growth at afirst crystal growth temperature wherein the island crystal regions eachinclude a boundary region; continuing growth of the island crystalregions at a second crystal growth temperature lower than the firstcrystal growth temperature while bonding of the boundary regions of theisland crystal regions occurs; and forming a semiconductor device on alayer of the nitride semiconductor formed by bonding the boundaryregions of the island crystal regions.
 32. The method according to claim31, wherein the semiconductor device is selected from the groupconsisting of a semiconductor light emitting device and a semiconductortransistor device.
 33. The method according to claim 32, wherein thesemiconductor light emitting device is selected from the groupconsisting of a semiconductor light emitting diode and a semiconductorlaser device.